1. Field of the Invention
This invention is directed to a method and apparatus for accurately determining the count per unit volume of particles suspended in a fluid containing one or more undesirable particles. More particularly, the invention pertains to determining the count per unit volume of blood cells in a flow cytometric system wherein the count is corrected for air bubbles present in the blood suspension medium.
2. Description of the Prior Art
The use of flow cytometry for counting blood cells has been increasing over the past several years. A fundamental consideration in the accuracy of blood cell counting is that the suspension of blood cells, if allowed to sit unmixed, will become spatially inhomogeneous due to sedimentation or sticking to the container walls. This occurs due to the fact that the blood cells are in general denser than the fluid they are suspended in and may also have an electrostatic interaction with the walls of the container. This inhomogeniety may interfere with the accuracy of the blood cell counting.
In order to solve this problem, the suspension of blood cells is mixed prior to the cells being passed through the counting mechanism. However, if the suspension is mixed, air bubbles will be introduced into the fluid which may interfere with the accuracy of the blood cell count. This interference is generally caused by the trapped air increasing the fluid volume measured by the system which erroneously lowers the cell count and also by small air bubbles which may be counted as cells erroneously increasing the cell count.
Heretofore, there has not been a system for solving the substantial problems in counting blood cells in a flow cytometer. At present, if the suspension is sampled during or shortly after mixing, error due to air is introduced and if counting is delayed to remove the air, cell loss due to sedimentation or sticking to the container walls is observed.